It has been recognized that activity of the sympathetic nerves to the kidneys contributes to essential hypertension, which is the most common form of hypertension. Sympathetic stimulation of the kidneys may contribute to hypertension by several mechanisms, including the stimulation of the release of renin (which results in production of angiotensin II, a potent vasoconstrictor), increased renal reabsorption of sodium, at least in part related to increased release of aldosterone (which increases blood volume and therefore blood pressure), and reduction of renal blood flow, which also results in angiotensin II production.
Since the 1930s it has been known that injury or ablation of the sympathetic nerves in or near the outer layers of the renal arteries can dramatically reduce high blood pressure. As far back as 1952, alcohol has been used for tissue ablation in animal experiments. Specifically Robert M. Berne in “Hemodynamics and Sodium Excretion of Denervated Kidney in Anesthetized and Unanesthetized Dog” Am J Physiol, October 1952 171:(1) 148-158, describes applying alcohol on the outside of a dog's renal artery to produce denervation.
Ablation of renal sympathetic nerves to treat drug-resistant hypertension is now a proven strategy [Symplicity-HTN-2 Investigators, Lancet 2010]. In order for the procedure to be successful, renal nerves need to be ablated such that their activity is significantly diminished. One drawback of ablation procedures is the inability for the physician performing the procedure to ascertain during the procedure itself that the ablation has been successfully accomplished. The reason for this is that the nerves cannot be visualized during the procedure; therefore, the procedure must be performed in a “blind” fashion. The ablation procedure is invasive, requiring catheterization of the femoral artery, advancement of a catheter into the renal artery, administration of iodinated contrast agents, and radiation exposure. Furthermore, procedural success with currently available devices is far from universal. In a randomized, controlled clinical trial using radiofrequency ablation, 16% of patients failed to achieve even a 10 mmHg reduction in systolic blood pressure and 61% did not achieve a goal systolic blood pressure of <140 mmHg [Symplicity-HTN-2 Investigators, Lancet 2010].
The procedure must be performed in a catheterization laboratory or operative-type suite. The benefit-risk of this invasive procedure as well as its cost-benefit would be enhanced if procedural success could be assessed during the procedure. Assessing the technical success of the procedure during the procedure would allow the physician to perform additional ablation attempts and/or to adjust the technique as needed, which, in turn is expected to improve efficacy and to reduce the need to bring the patient back for a second procedure at additional cost and risks to the patient. The desired effect of renal sympathetic nerve ablation procedure is a lowering of blood pressure, with consequent reduction in the need for chronic antihypertensive drug treatment. Since the blood pressure lowering effect of the treatment does not occur immediately, the blood pressure measured in the catheterization laboratory also cannot act as a guide to the technical success of the procedure.
There are currently two basic methods to ablate renal sympathetic nerves: energy-based neural damage resulting from radiofrequency or ultrasonic energy delivery and chemical neurolysis. Both methods require percutaneous insertion of a catheter into the renal arteries. Radiofrequency-based methods transmit radiofrequency energy through the renal artery wall to ablate the renal nerves surrounding the blood vessel. Chemical neurolysis uses small gauge needles that pass through the renal artery wall to inject a neurolytic agent directly into the adventitial and/or periadvential area surrounding the blood vessel, which is where the renal sympathetic nerves entering and leaving the kidney (i.e., afferent and efferent nerves) are located.
Recent technology for renal denervation include energy delivery devices using radiofrequency or ultrasound energy, such as Simplicity™ (Medtronic), EnligHTN™ (St. Jude Medical) and One Shot system from Covidien, all of which are RF ablation catheters. There are potential risks using the current technologies for RF ablation to create sympathetic nerve denervation from inside the renal artery. The short-term complications and the long-term sequelae of applying RF energy from the inner lining (intima) of the renal artery to the outer wall of the artery are not well defined. This type of energy applied within the renal artery, and with transmural renal artery injury, may lead to late stenosis, thrombosis, renal artery spasm, embolization of debris into the renal parenchyma, or other problems related to the thermal injury to the renal artery. There may also be uneven or incomplete sympathetic nerve ablation, particularly if there are anatomic anomalies, or atherosclerotic or fibrotic disease in the intima of the renal artery, the result being that there is non-homogeneous delivery of RF energy. This could lead to treatment failures, or the need for additional and dangerous levels of RF energy to ablate the nerves that run along the adventitial plane of the renal artery. Similar safety and efficacy issues may also be a concern with the use of ultrasound. The Simplicity™ system for RF delivery also does not allow for efficient circumferential ablation of the renal sympathetic nerve fibers. If circumferential RF energy were applied in a ring segment from within the renal artery (energy applied at intimal surface to damage nerves in the outer adventitial layer) this could lead to even higher risks of renal artery stenosis from the circumferential and transmural thermal injury to the intima, media and adventitia. Finally, the “burning” of the interior wall of the renal artery using RF ablation can be extremely painful. The long duration of the RF ablation renal denervation procedure requires sedation and, at times, extremely high doses of morphine or other opiates, and anesthesia, close to general anesthesia, to control the severe pain associated with repeated burning of the vessel wall. This is especially difficult to affect with any energy based system operating from inside the renal artery as the C-fibers which are the pain nerves are located within or close to the media layer of the artery. Thus, there are numerous and substantial limitations of the current approach using RF-based renal sympathetic denervation. Similar limitations apply to ultrasound or other energy delivery techniques.
The Bullfrog® micro infusion catheter described by Seward et al in U.S. Pat. Nos. 6,547,803 and 7,666,163, which uses an inflatable elastic balloon to expand a single needle against the wall of a blood vessel, could be used for the injection of a chemical ablative solution such as guanethidine or alcohol but it would require multiple applications as those patents do not describe or anticipate the circumferential delivery of an ablative substance around the entire circumference of the vessel. The greatest number of needles shown by Seward is two, and the two needle version of the Bullfrog® would be hard to miniaturize to fit through a small guiding catheter to be used in a renal artery particularly if needles of adequate length to penetrate to the periadventitia were used. If only one needle is used, controlled and accurate rotation of any device at the end of a catheter is difficult at best and could be risky if the subsequent injections are not evenly spaced. This device also does not allow for a precise, controlled and adjustable depth of delivery of a neuroablative agent. This device also may have physical constraints regarding the length of the needle that can be used, thus limiting the ability to inject agents to an adequate depth, particularly in diseased renal arteries with thickened intima. All of these limitations could lead to incomplete denervation and treatment failure. Another limitation of the Bullfrog® is that inflation of a balloon within the renal artery can induce transient renal ischemia and possibly late vessel stenosis due to balloon injury of the intima and media of the artery, as well as causing endothelial cell denudation.
Jacobson and Davis in U.S. Pat. No. 6,302,870 describe a catheter for medication injection into the interior wall of a blood vessel. While Jacobson includes the concept of multiple needles expanding outward, each with a hilt to limit penetration of the needle into the wall of the vessel, his design depends on rotation of the tube having the needle at its distal end to allow it to get into an outwardly curving shape. The hilt design shown of a small disk attached a short distance proximal to the needle distal end has a fixed diameter which will increase the total diameter of the device by at least twice the diameter of the hilt so that if the hilt is large enough in diameter to stop penetration of the needle, it will significantly add to the diameter of the device. Using a hilt that has a greater diameter than the tube, increases the device profile, and also prevents the needle from being completely retracted back inside the tubular shaft from which it emerges, keeping the needles exposed and potentially allowing accidental needlestick injuries to occur. For either the renal denervation or atrial fibrillation application, the length of the needed catheter would make control of such rotation difficult. In addition, the hilts, which limit penetration, are a fixed distance from the distal end of the needles. There is no built in adjustment on penetration depth which may be important if one wishes to selectively target a specific layer in a vessel or if one needs to penetrate all the way through to the volume of tissue outside of the adventitia in vessels with different wall thicknesses. Jacobson also does not envision use of the injection catheter for denervation. Finally, FIG. 3 of the Jacobson patent shows a sheath over expandable needles without a guide wire, and the sheath has an open distal end which makes advancement through the vascular system more difficult. Also, because of the hilts, if the needles were withdrawn completely inside of the sheath they could get stuck inside the sheath and be difficult to push out. The complexity of this system might also lead to inadequate, or incomplete renal denervation.
McGuckin in U.S. Pat. No. 7,087,040 describes a tumor tissue ablation catheter having three expandable tines for injection of fluid that exit a single needle. The tines expand outwardly to penetrate the tissue. The McGuckin device has an open distal end that does not provide protection from inadvertent needle sticks from the sharpened tines. In addition, the McGuckin device depends on the shaped tines to be of sufficient strength so that they can expand outwardly and penetrate the tissue. To achieve such strength, the tines would have to be so large in diameter that severe extravascular bleeding would often occur when the tines would be retracted back following fluid injection for a renal denervation application. There also is no workable penetration limiting mechanism that will reliably set the depth of penetration of the distal opening from the tines with respect to the interior wall of the vessel, nor is there a preset adjustment for such depth. For the application of treating liver tumors, the continually adjustable depth of tine penetration may make sense since multiple injections at several depths might be needed. However, for renal denervation, the ability to accurately adjust the depth or have choice of penetration depth when choosing the device to be used is important so as to not infuse the ablative fluid too shallow and injure the media of the renal artery or too deep and thus miss the nerves that are in the adventitial and periadventitial layers of the renal artery.
Fischell et al in U.S. patent application Ser. Nos. 13/216,495, 13/294,439 and 13/342,521 describe several methods of using expandable needles to deliver ablative fluid into or deep to the wall of a target vessel. Each of these applications is hereby incorporated by reference in its entirety. There are two types of embodiments of application Ser. Nos. 13/216,495, 13/294,439 and 13/342,521, those where the needles alone expand outwardly without support from any other structure and those with guide tubes that act as guiding elements to support the needles as they are advanced into the wall of a target vessel. The limitation of the needle alone designs are that if small enough diameter needles are used to avoid blood loss following penetration through the vessel wall, then the needles may be too flimsy to reliably and uniformly expand to their desired position. The use of a cord or wire to connect the needles together in one embodiment helps some in the area. The use of guide tubes as described in the Fischell application Ser. Nos. 13/294,439 and 13/342,521 greatly improves this support, but the unsupported guide tubes themselves depend on their own shape to ensure that they expand uniformly and properly center the distal portion of the catheter. Without predictable catheter centering and guide tube expansion it may be challenging to achieve accurate and reproducible needle penetration to a targeted depth. More recently in U.S. patent application Ser. No. 13/752,062, Fischell et al describe self-expanding and manually expandable ablation devices that have additional structures to support the needle guiding elements/guide tubes. The Ser. No. 13/752,062 designs for a Perivascular Tissue Ablation Catheter (PTAC) will be referenced throughout this disclosure.
While the prior art has the potential to produce ablation of the sympathetic nerves surrounding the renal arteries and thus reduce the patient's blood pressure, none of the prior art includes sensors or additional systems to monitor the activity of the sympathetic nerves being ablated. Such measurement would be advantageous as it could provide immediate feedback relative to the effectiveness of the ablation procedure and indicate if an additional ablation administration may be needed. For example, additional energy delivery or additional ablative fluid delivery could be administered if the nerves are still conducting (electrical) activity.
It is technically feasible to measure renal sympathetic activity directly or indirectly in vivo using several methods. Such measurements have been accomplished in unrestrained conscious mice [Hamza and Hall, Hypertension 2012], dogs [Chimushi, et al. Hypertension 2013], and rabbits [Doward, et al. J Autonomic Nervous System 1987].
In the study by Hamza and Hal, an electrode was surgically placed directly on the renal nerves and left in place while recordings were made over up to 5 days. The recordings of renal sympathetic nerve activity were confirmed by observations of appropriate responses to conditions of rest and activity, pharmacologic manipulation of blood pressure with sodium nitroprusside and phenylephrine, and by neural ganglionic blockade. Doward, et al also used surgical placement of an electrode to directly measure renal sympathetic nerve activity. The recordings of renal sympathetic nerve activity were confirmed by observations of appropriate responses to baroreceptor stimulation, angiotensin, central and peripheral chemoreceptors. In the study by Chimushi, renal sympathetic nerves were stimulated from within the renal artery and evidence of activity was indirectly evaluated based on blood pressure response to neural stimulation.